Theoretical Simulation Study Of Seismoelectric Wavefields In Seabed And Borehole

Seabed and borehole interfaces can be regarded as fluid/porous-medium interfaces.Elastic waves passing through the seabed and the borehole give rise to seismoelectric coupled waves which couple elastic waves and electromagnetic fields due to seismoelectric effect of electric double layer in the porous medium.Seismoelectric coupled waves combine the spatial resolution of elastic waves and the reservoir identification ability of electromagnetic waves,so that seismoelectric wavefields attract attention from oil-gas exploration fields.In recent years,there have been many theoretical studies on seismoelectric wavefields in the seabed and borehole,but the propagation law of seismoelectric wavefields has not been fully recognized due to the complexity of the seismoelectric effect of electric double layer.This thesis derives respectively seismoelectric wavefields in the seabed and the borehole,based on the half-space and the cylindrical radial stratified models of the fluid/porous-medium interface,and the propagation law of seismoelectric wavefields is analyzed.Based on a half-space model of a fluid/porous-medium interface,expressions of seismoelectric reflection and transmission coefficients are derived by simplified Pride equations.The expressions establish an intuitive physical relationship between reflected and transmitted electromagnetic waves from the interface and elastic waves in the porous medium.The expressions show that the elastic waves affect the reflected and transmitted electromagnetic waves through the product of the ratio of the seepage displacement to solid displacement and the transmission coefficient.Besides,simplified formulas of seismoelectric reflection and transmission coefficients are obtained by using the electromagnetic premise condition proposed in this thesis that the electromagnetic wavenumber is far less than the interface tangential wavenumber.The simplified formulas show that the viscosity coefficient,electrokinetic coupling coefficient,conductivity and permeability appear as a whole.Calculations show that the effect of fast compressional waves on reflected and transmitted electromagnetic waves can be ignored.The simplified formulas provide convenience for the inversion of reservoir parameters.Based on the model of an infinite plane rigid baffle embedded with a circular piston source(baffled piston source,BPS),the conical expansion is deduced for radiated acoustic waves from BPS incident on any interface when the piston and interface are not parallel.Based on a half-space model of a fluid/solid interface,the expressions of interface reflected and transmitted acoustic fields caused by radiated waves from BPS for the piston non-parallel with the interface are derived by using the conical expansion and boundary conditions.Besides,the reflected acoustic field expression is simplified through saddle point method and its simplified formula is obtained.The formula is not only simple in the calculation but also clear in the physical meaning: reflected acoustic waves caused by radiated waves from BPS can be regarded as the product of axisymmetric directional spherical-waves excited by the mirror image of BPS and the reflection coefficient.The effect of the angle between the piston and interface on the reflected acoustic fields is analyzed.Results show that the reflected acoustic fields and its directivity are sensitive to the angle between the piston and interface for the large piston source(the piston radius i s larger than the acoustic wavelength in the fluid),while the reflected acoustic fields and its directivity are not sensitive to the angle between the piston and interfac e for the small piston source.The simplified formula provides convenience for marine seismic exploration and ultrasonic detection of underwater targets.Based on a half-space model of a fluid/porous-medium interface,the expressions of interface reflected and transmitted electromagnetic fields caused by radiated waves from BPS for the piston non-parallel with the interface are derived by using the conical expansion and boundary conditions.Besides,the simplified algorithm of the reflected electromagnetic field is given by Taylor expansion method.The simplified algorithm does not need to calculate integrals of Bessel functions in the Elliptic surface and thus greatly improves calculation efficiency.The effect of the angle between the piston and interface on the electric fields of reflected electromagnetic waves near the interface is analyzed.Results show that the electric field accompanying Stoneley wave is sensitive to the angle between the piston and interface.Amplitudes of radial,circumferential and axial electric fields increase with the angle between the piston and interface.The simplified algorithm provides convenience for marine seismoelectric exploration.Based on the fluid-filled borehole surrounded by radial layered formation,three types of borehole seismoelectric wavefields are derived.The effect of salinity interfaces on borehole seismoelectric wavefields is analyzed and the salinity interface condition for the appearance of large amplitude electromagnetic head wave(EH wave)is revealed,so that the phenomenon of large amplitude EH wave in previous field observation is explained.The relationship is analyzed between the amplitude ratio of the EH wave to the electric field accompanying Stoneley wave(ESt wave)and the salinity ratio on two sides of the interface.Results show that the amplitude ratio is determined by the salinity ratio rather than the salinity itself and the amplitude ratio increases with the salinity ratio.The high amplitude ratio of EH to ESt waves can be used to detect high salinity interface and borehole mudcake,and to invert the salinity ratio at the interface,which provides idea for the application of the EH wave.